A water-cooling type cooling device that employs a radiator is generally used in an automobile engine in order to cool same. Further, conventionally, with the object of improving the fuel consumption of the automobile, this type of cooling device employs a control valve, such as a thermostat, for example, for adjusting the amount of cooling water circulated to the radiator so as to permit control of the temperature of the cooling water introduced to the engine. Known examples of such thermostats include those which employ a thermally expanding body as a temperature sensor or those which are electrically controlled, and so forth.
A thermostat of this kind is constituted such that the valve portion thereof is interposed in part of a cooling water passage such that when the cooling water temperature is low, the valve portion is closed so that cooling water is circulated via a bypass passage without passing through the radiator, and, when the cooling water temperature is high, the temperature of the cooling water can be controlled to the required state by closing the valve portion so that the cooling water is circulated via the radiator.
Further, it is generally known that the fuel consumption of the automobile is improved by reducing the cooling water temperature when the engine is running with a high load and raising the cooling water temperature when the load is low.
In view of this situation, most recently, electronic-control type valves, that is, electronically controlled thermostats have been widely adopted in order to provide the optimum water temperature for improving automobile fuel consumption. Such an electronically controlled thermostat controls the cooling water temperature by optionally controlling the opening ratio of the valve portion and controlling a cooling fan that is attached to the radiator, whereby appropriate control of the cooling water temperature is possible.
This is because a control device (engine control module) that variably controls the above-described electronically controlled thermostat is capable of performing control also through the addition of detected information such as information on a variety of parameters of the engine control unit, such as the cooling water temperature, the outside air temperature, the engine revolution speed, and throttle opening ratio, for example.
A multiplicity of different types of thermostats has been proposed conventionally as means for improving fuel consumption by controlling the cooling water temperature at the required state.
For example, Japanese Patent Application No. 10-227215 discloses, as an example of an engine water temperature control device, a technology according to which “it is judged whether or not a temperature detected by a water temperature sensor exceeds a target temperature, and, when this temperature exceeds the target temperature, the cooling water control valve portion opens at an opening ratio based on the detected temperature, and, when the opening ratio is above a set value, the fan motor of the cooling fan is caused to rotate at a rotation speed that corresponds with the opening ratio to forcedly cool the radiator cooling water”.
However, the above-described conventional cooling water temperature control has posed the following problems. That is, where the conventional cooling water temperature control is concerned, unavoidable problems include the cooling water temperature control being performed unnecessarily due to problems such as responsiveness, the target temperature being overshot or undershot in attempts to set the cooling water temperature at the target temperature, and the occurrence of futile water temperature changes (so-called temperature hunting) in repeating the valve operation many times over until the target temperature is reached, these problems being the cause of fuel consumption degradation.
Further, there is the drawback that, because a temperature hunting phenomenon caused by an excessive amount of cooling water flowing when the thermostat valve is opened is readily produced, the tracking and stability of the cooling water temperature are poor due to this temperature hunting, and therefore the stabilized output when the engine load is high is undesirable.
There is also the inconvenience that, because a high water temperature set value is set in consideration of a stable machine due to the above-described overshooting, high water temperature control up to the limits of the permitted range cannot be performed.
There is also the problem that control at a higher water temperature cannot be performed because the stability and tracking of the cooling water temperature are poor due to the variation in the water temperature at the radiator outlet and the variation in the radiator flow rate that is caused by fluctuations in the heat generation of the engine and in the rotation speed of the water pump.
There is also the problem that, because the cooling fan operates after a transition to a low water temperature when the radiator outlet water temperature is high has been determined, the operational timing of the cooling fan is delayed and hence the change to a low cooling water temperature is delayed.
Further, where conventional control is concerned, it is necessary to detect the radiator outlet cooling water temperature in order to make the cooling water temperature linear or close to the ideal temperature. For this reason, a water temperature sensor, water temperature switch, or the like, must be provided at the radiator outlet, and hence costs are high.
In addition, with the above-described conventional cooling water temperature control, even if control to establish a set water temperature has been possible in tests, the actual vehicle is affected by a variety of external factors such as the outside air temperature and inside cabin temperature, which is associated with a deterioration in control. There are therefore also problems such as it not being possible to obtain ideal results.
The present invention was conceived in view of this situation, and has, as an object, to provide a control method for an electronically controlled thermostat control method that makes it possible to set the cooling water temperature appropriately and efficiently in accordance with the engine load when the engine is running, that is also superior in terms of responsiveness and cooling water temperature stability, that appropriately controls the cooling water temperature to a high water temperature or a low water temperature without there being the risk of overshooting or undershooting, temperature hunting, and so forth, and that allows an improvement in the fuel consumption to be achieved more reliably and substantially over the whole range of running states.